How to install composite decking

Where do you start laying composite deck boards?

Start from a straight edge and let the cut board end up where nobody looks. Ekodeck’s installer starts at the outer edge and works back to the house, so the board you have to rip down finishes against the wall. A UK installer working with a different system starts at the house wall and works out. Both agree on one thing: the first board decides where every later board sits, so set it with a string line and keep checking square as you go.

Lay a few boards before you fully tighten. ClickDeck suggests 6 to 10, and Ekodeck’s installer checks the deck is still square every so often rather than just at the start.

What does the frame need first?

The frame sets joist centres, ground clearance and drainage, and each maker publishes its own.

Boards run over at least three joists. ModWood says good ventilation is essential and that deck ends must not be sealed off with garden beds, trees or solid objects, because lack of ventilation causes swelling and failure. Boards must never sit directly on a solid surface.

Deck over timber? Look at the joists and bearers guide for how the frame is made. Over concrete, the aluminium frame page covers low-profile systems.

How big should the gaps be?

Side gaps are usually set by the clip. End gaps depend on temperature, board length and the maker, and mixing them up is the commonest way to buckle a deck.

• ModWood requires a 4 mm side gap and calls it critical, because wood fibres swell slightly after laying. Its older sheet gives 2 mm at butt joins and 6 mm to a solid structure.
• Ekodeck’s Quickfix clip sets 3 mm or 6 mm automatically, and the installer leaves at least 3 mm at a wall.
• Futurewood gives 2 to 3 mm side gaps, 2 to 5 mm at butt joins and 5 mm at a wall.
• NewTechWood’s hidden fixings give 3 mm or 5.5 mm.

Temperature changes the end gap. Futurewood says a 5.4 m board grows about 2.5 mm per 10 degrees of temperature rise, and that boards over 3 m shouldn’t be butt joined at all. NewTechWood publishes a table: a 5.4 m board needs 8.6 mm at 10 degrees, 5.4 mm at 25 degrees and 2.2 mm at 40 degrees, and the figure halves if you leave a gap at both ends.

When is it too hot or cold to install?

NewTechWood says the ideal installation temperature is 5 to 25 degrees, acclimatise boards for at least 2 days and don’t install in frost or heat. ModWood says to fix when the air is below 25 degrees and the boards aren’t in full sun. Futurewood says to cut and install below 30 degrees, with all boards at a similar temperature. A US installer makes the same point: hot boards are longer, so keep them in the shade or wet them down before fixing.

How do you fix the boards?

Use the maker’s clips. Ekodeck’s installer fits one locking clip per board on the middle joist, standard clips on the rest, and one screw per clip. Futurewood does the same with a locking clip on the joist nearest the board’s centre to stop creep. One installer says to stop at the first firm resistance when tightening clip screws, because overdriving cracks the clip.

The last board has no room for a clip. Ekodeck’s installer pre-drills, countersinks and screws it from the top. NewTechWood doesn’t recommend face-fixing a whole deck, and where you must, pre-drill slightly larger than the screw and keep screws 38 mm from board ends and edges on fascia and picture-frame boards. ModWood says pre-drilling and countersinking is essential for face fixings.

What changes on a steel frame?

Steel moves with the temperature in a different way from composite, so fixings matter. Ekodeck pre-drills (3 mm for steel under 1.5 mm thick, 3.5 mm for 1.5 to 2.4 mm, 3.75 mm above that) and pop-rivets clips on steel frames. ModWood uses tek screws and prefers a concealed clip. Futurewood’s self-drilling screws are rated for steel up to 1.6 mm, and NewTechWood’s Metal Fix screws suit steel up to 2 mm.

What about corners and cut-outs?

One installer who built a picture-frame border on an uneven slab did the corners by cutting the first board at 45 degrees and marking the second against it, which beat guessing the angle. They planned the deck in modules on paper, built and levelled the perimeter frame first, and ran cables for lights before laying boards.

Seal cut ends if the maker advises it. NewTechWood recommends a clear sealant on cut ends.